Add --user option to create user namespace

[JonathonReinhart]

This adds a -U/--user option which uses CLONE_NEWUSER to create a new user namespace, which would avoid the need to run as root.

However, it is incomplete because it does not set any uid/gid mappings, causing this bad behavior due to invalid uid/gid:

/proc/sys/net/ipv4/ping_group_range: 65534	65534
  ->  65535 65535
Child function raised exception:
OSError: [Errno 22] Invalid argument